Temperature-dependent kinetics of the atmospheric reaction between CH2OO and acetone.

Abstract

Criegee intermediates are important oxidants produced in the ozonolysis of alkenes in the atmosphere. Quantitative kinetics of the reactions of Criegee intermediates are required for atmospheric modeling. However, the experimental studies do not cover the full relevant range of temperature and pressure. Here we report the quantitative kinetics of CH2OO + CH3C(O)CH3 by using our recently developed dual strategy that combines coupled cluster theory with high excitation levels for conventional transition state theory and well validated levels of density functional theory for direct dynamics calculations using canonical variational transition theory including tunneling. We find that the W3X-L//DF-CCSD(T)-F12b/jun-cc-pVDZ electronic structure method can be used to obtain quantitative kinetics of the CH2OO + CH3C(O)CH3 reaction. Whereas previous investigations considered a one-step mechanistic pathway, we find that the CH2OO + CH3C(O)CH3 reaction occurs in a stepwise manner. This has implications for the modeling of Criegee-intermediate reactions with other ketones and with aldehydes. In the kinetics calculations, we show that recrossing effects of the conventional transition state are negligible for determining the rate constant of CH2OO + CH3C(O)CH3. The present findings reveal that the rate ratio between CH2OO + CH3C(O)CH3 and OH + CH3C(O)CH3 has a significant negative dependence on temperature such that the CH2OO + CH3C(O)CH3 reaction can contribute as a significant sink for atmospheric CH3C(O)CH3 at low temperature. The present findings should have broad implications in understanding the reactions of Criegee intermediates with carbonyl compounds and ketones in the atmosphere.